In the manufacture of paper tissues, the wet web of fibers, a.k.a. the wet paper sheet, is formed on a fourdrinier or crescent former or twin wire, then transferred via a felt to a steam heated metal cylinder and dried thereon. The steam heated metal cylinder is typically known as the creping cylinder or “Yankee Dryer”. As the wet web of fibers rotates on the cylinder, much of the water is driven off leaving a web of fibers with from about 50 to about 99 weight percent solids. A metal blade, known as a creping blade, is then used to remove the web of fibers and in the process of removing the web the metal blade compacts the sheet in the machine direction which produces a folding action. This removal and compacting process is known as “creping”. Creping causes the paper sheet to wrinkle or pucker. Creping often destroys a large number of fiber to fiber bonds in the paper sheet thereby imparting qualities of bulk, stretch, absorbency and softness characteristics to the tissue paper being manufactured.
In tissue making it is normal practice to spray a dilute adhesive solution through a spray boom onto the heated metal surface of a creping cylinder to aid in adhesion of a web of fibers to the creping cylinder for drying and subsequent creping. This adhesive material provides adequate adhesion of the web of fibers to the creping cylinder which enhances the manufacture of quality tissue, helps protect the dryer from excessive wear, provides lubrication for the doctor blades and is soft enough to allow doctor blade tip penetration for good creping. After encountering the layer of adhesive on the creping cylinder, typically the web of fibers is adhered to the cylinder using a pressure roll or suction pressure roll that is positioned such that the web of fibers encounters the pressure roll nip (the pressure roll nip being the point of contact between the pressure roll and the creping cylinder) at approximately the same time that the web of fibers encounters the layer of adhesive. The sheet then continues around the heated cylinder to be creped off with a metallic blade. In the creping process valued attributes such as softness, absorbency and bulk are built into the sheet. After the web of fibers has been removed from the creping cylinder by the creping blade, state of the art techniques currently call for spraying the surface of the creping cylinder again with the dilute adhesive solution and the creping process is continued.
After the web of fibers has been removed from the creping cylinder by the creping blade, some material, which may include heat-solidified adhesive and stray fibers, is typically left on the surface of the dryer. The material left on the surface of the dryer tends to adhere to the surface and the build-up eventually gets large enough to be described as a “deposit”. Deposits at the edge of the fiber web due to residual adhesive solution being baked onto the cylinder are a major problem because the presence of the deposit leads to uneven coating of the cylinder with the fiber mat and that can lead to doctor blade chattering and poor runnability and that can lead to unwanted breakage of the fiber mat.
Another known problem with the creping process is that the edges of the creping cylinder that have no web of fibers or felt in contact tends to be at a temperature higher than the temperature at the center part of the creping cylinder. The resulting temperature gradient that is in existence across the cylinder can cause uneven processing of the wet mat of fibers.
With each section of the creping cylinder having a different performance requirement and each section of the creping cylinder having an operating temperature range that is different from the adjacent section it then becomes apparent that it is difficult to maintain good runnability of the creping cylinder.
Past attempts to improve runnability by adding one or more modifiers to the dilute adhesive solution have not been optimal because there has yet to be one modifier identified that can improve runnability across the entire length of the creping cylinder. It would be desirable then to have a way of improving the performance of a creping cylinder to correct the problems associated with the temperature gradient across the creping cylinder and the problems associated with deposits left on the creping cylinder.